Process for the preparation of expandable polyvinylarene particles

ABSTRACT

Process for the preparation of expandable polyvinylarene particles in which compact polyvinylarene particles are impregnated by an inorganic N2- and/or O2-containing gas at a temperature below 95° C. and at a pressure of 100 to 2,000 kPa gauge.

The present invention relates to a process for the preparation ofexpandable polyvinylarene particles and to a process for expanding theseparticles and to the preparation of foamed articles from the expandedparticles thus expanded.

For many years it has been known that particles of polyvinylarenes, suchas polystyrene, can be rendered expandable and that the particles thusobtained can be used in the preparation of foamed articles. In thisrespect reference is made to, e.g., U.S. Pat. No. 2,681,321 whichdiscloses a process in which polystyrene particles are exposed to liquidhydrocarbons and treated such that the liquid hydrocarbon is dispersedin the polystyrene particles. Particles thus prepared contain generally4 to 8% wt of such liquid hydrocarbon, such as butane, n-pentane ormixtures of pentanes. These particles can then be expanded to beads witha reduced density. Apparent densities for packaging particles typicallyare 20 to 60 kg/m³. Once expanded, the particles are fused in asteam-heated mould to yield a foamed article of a desired shape.

One of the factors that influence the expansion of the polystyreneparticles is the amount of hydrocarbon blowing agent. From Kirk Othmer,Encyclopedia of Chemical Technology, third edition, Volume 21, page 838,it can be read that the density of particles containing 5.7% wtn-pentane is typically 1080 kg/m³, compared to a value of 1050 kg/m³ forpure polystyrene beads and compared with a calculated density of 1020kg/m³ for a simple mixture in which the n-pentane is dissolved inpolystyrene. If all pentane would be in voids the calculated densitywould be 1120 kg/m³. Thus it has been suggested that part of thehydrocarbon blowing agent is present in little voids in the polystyrene.The skilled man will appreciate that the above densities are particledensities, which can be recalculated to apparent densities. A particledensity of 1050 kg/m³ corresponds to an apparent density of around 680kg/m³.

A drawback of the present practice is that during the transport andstorage of the unexpanded particles hydrocarbons may evaporate from theparticles, in particular from the voids. When the particles aretransported and/or stored at varying temperatures and/or duration, theamounts of e.g. pentane retained may vary significantly. Apart fromextra safety measures that have to be taken during transport, likegas-tight packaging, it will be appreciated that such a variation mayhave an effect on the resulting foam obtained after expansion.

Furthermore, the expansion process itself also causes that hydrocarbonsoriginally present in the unexpanded particles are emitted into theenvironment. In order to reduce the emissions, complicated equipment hasbeen developed to collect the emitted hydrocarbons for further handling,i.e. combustion. This equipment is to be installed in the facilities ofthe end-user of the particles, i.e. the customer who produces the foamedarticles. This requires additional expertise and investments with thesecustomers.

U.S. Pat. No. 5,358,675 discloses a process in which a polymericmaterial, e.g. polystyrene, is combined with another second phasematerial, e.g. a rubber, in an extruder at elevated temperature. Themixture is impregnated with nitrogen at pressures ranging from 800 up to2000 psi (5,500-13,800 kPa). The presence of the rubber is necessary toobtain a suitable cell size above 10 μm. If no rubber is added the cellsobtained are too small. Although the process of this document overcomesthe environmental problem, it requires excessive pressures. It has nowbeen found that particles of polyvinylarene can be expanded sufficientlyby avoiding these high pressures.

Accordingly, the present invention provides a process for thepreparation of expandable polyvinylarene particles in which compactpolyvinylarene particles are impregnated by an inorganic N₂- and/orO₂-containing gas at a temperature below 95° C. and at a pressure of 100to 2,000 kPa gauge.

Compact polyvinylarene particles suitably have an apparent density ofmore than 600 kg/m³, more suitably more than 620 kg/m³. Generally, theapparent density of these particles will not exceed 700 kg/m³.

Typically, the compact polyvinylarene particles of the present inventionare not expandable before they are impregnated with an N₂- and/orO₂-containing gas. In practice, this means that they contain less than0.5% by weight, based on the amount of vinylarene, of volatile organiccompounds, more preferably less than 0.1% by weight. Most preferably,the compact polyvinylarene particles do not contain any volatile organiccompound at all. The amount of water in the compact polyvinylareneparticles will suitably not exceed 1% by weight, based on the amount ofvinylarene.

The compact polyvinylarene particles may be prepared by various methods,including bulk polymerization, solution polymerisation, and suspensionpolymerization or mixtures of these methods. Preferably thepolymerization is carried out in a bulk polymerization or suspensionpolymerization process. In bulk techniques, the polymerisation iscarried out in a conventional way to obtain small particles (nibs).Suspension polymerisation includes the suspending of vinylarene monomerin an aqueous phase and polymerising the suspended droplets. Bothmethods ensure that sufficient voids are present in the polymerparticles into which the inorganic N₂- and/or O₂-containing gas can beimpregnated.

Suspension polymerisation is suitably carried out in the presence ofsuspension stabilisers. Suitable suspension stabilisers are well knownin the art and comprise poly(vinyl alcohol), gelatine, agar, polyvinylpyrrolidine, polyacrylamide, inorganic stabilisers such as alumina,bentonite, magnesium silicate or phosphates, like tricalciumphosphateand/or disodiumhydrogen phosphate, optionally in combination with any ofthe stabilising compounds mentioned earlier. The amount of stabilisermay suitably vary from 0.1 to 0.9% wt, based on the weight of theaqueous phase.

The suspension polymerisation is suitably carried out at two temperaturestages, in which the temperature in the first stage is from 85 to 110°C. and in the second stage is from 115 to 140° C.

The polymerization per se is well known in the art. It may be initiatedthermally, via free-radical polymerization or via anionicpolymerisation. Although all methods are equally possible, preference isgiven to free-radical polymerization. Suitable free-radical initiatorscan be selected from conventional initiators for free-radicalpolymerization. They include in particular organic peroxy compounds,such as peroxides, peroxy carbonates and peresters. Typical examples ofthese peroxy compounds are C₆₋₂₀ acyl peroxides, such as decanoylperoxide, benzoyl peroxide, octanoyl peroxide, stearyl peroxide,peresters, such as t-butyl perbenzoate, t-butyl peracetate, t-butylperisobutyrate, t-butyl-peroxy-(2-ethylhexyl)carbonate, hydroperoxidesand dihydrocarbyl peroxides, such as those containing C₃₋₁₀ hydrocarbylmoieties, including di-isopropyl benzene hydroperoxide, di-t-butylperoxide, dicumyl peroxide or combinations thereof. Other initiatorsdifferent from peroxy compounds are also possible, e.g.,α,α′-azobisisobutyronitrile.

The vinylarene comprised in the polymer of the present process consistspreferably mainly of styrene. The polyvinylarene may contain up to 10%mole of another vinyl group containing monomer, such as acrylonitril,acrylic or methacrylic acid or esters, substituted styrene, such aschlorostyrene, or α-methylstyrene, or divinylbenzene. However,preferably the vinylarene in the polyvinylarene consists for more than99% mole of styrene. More preferably, the polyvinylarene is purepolystyrene.

It may be advantageous to polymerise the vinylarene monomers in thepresence of other polymers such as polyphenylene oxide. Suitablepolyphenylene oxides have been described in EP-A-350137, EP-A-403023 andEP-A-391499. The polyphenylene oxide is preferably present in an amountof between 1 and 30% wt, based on the amount of vinylarene monomers, andmay improve the rigidity of the polyvinylarene polymer.

The compact polyvinylarene particles may contain various conventionaladditives. Such additives include chain transfer agents, cross-linkingagents and nucleating agents. Suitable examples of chain transfer agentsare C₂₋₁₅ alkyl mercaptans such as n-dodecyl mercaptan, t-dodecylmercaptan, t-butyl mercaptan and n-butyl mercaptan. Other agents arepentaphenylethane and the dimer of α-methylstyrene. Examples ofcross-linking agents are butadiene and divinylbenzene. Nucleating agentsare agents that promote cell formation and are suitably used in anamount of 0.01 to 3% by weight, based on vinylarene, preferably in anamount of 0.05 to 2% by weight. Examples of nucleating agents are finelydispersed inorganic compounds, organic compounds and polymer particles.Examples are carbonates compounds such as calcium carbonate, sulphatecompounds such as barium sulphate and calcium sulphate, silicatecompounds such as talc, clay, magnesium silicate, amorphous silicaparticles, zeolites, diatomaceous earth, oxides such as magnesium oxide,and titanium oxide, mixtures of sodium bicarbonate with citric acid,organic bromide-containing compounds, naphthalene compounds, polycyclicaromatic hydrocarbons, carbon black, cokes, chars, graphite and diamonddust, paraffin and fatty acid derivatives such as stearate and palmitatecompounds. Examples of suitable polymer particles are polyvinylchloride,polypropylene, polyethylene, acrylonitril butadiene styrene rubber,styrene butadiene rubber, styrene/maleic anhydride copolymer andcellulose. Further examples include polar polymers as described in e.g.WO 98/01501 which comprise e.g. starch, and starch modified byesterification or etherification, emulsifiers as described e.g. WO98/01488 and WO 98/01489 which comprise bisalkylsulphosuccinates,sorbitol-C₈-C₂₀-carboxylates, and C₈-C₂₀-alkylxylene sulphonates.Particularly suitable as nucleating agent are polyethylene waxes havinga weight average molecular weight of 500 to 5,000, which are typicallyfinely divided through the polymer matrix in a quantity of 0.01-1.0% byweight, based on the amount of vinylarene, preferably from 0.1 to 0.5%by weight.

The compact vinylarene particles may also contain anti-static additives,flame retardants such as hex-abromocyclododecane, dyes, filler material,plasticizers, such as white oil, stabilisers and lubricants. Theparticles are suitably coated with coating compositions comprisingsilicones, silicates metal or glycerol carboxylates. Suitablecarboxylates are glycerol mono-, di- and tri-stearate, zinc stearate,and mixtures thereof. Examples of such compositions have been disclosedin GB patent No. 1,409,285. Instead of stearate, one may also usecitrate or palmitate. The coating compositions have been applied to theparticles via dry-coating in a ribbon blender or via a slurry orsolution in a readily vapourizing liquid.

The compact polyvinylarene particles are impregnated with an inorganicN₂- and/or O₂-containing gas, at a temperature below 95° C. and at apressure of 100 to 2,000 kPa gauge. These gases suitably contain morethan 90% by volume, based on the volume of the gas, of N₂ and/or O₂,more suitably more than 95% by volume. “Inorganic” means that the gasesaccording to the present invention may contain at most 1% by volume,based on the volume of the gas, of organic compounds, preferably at most0.5% by volume. Most preferably, these gases do not contain any organiccompounds.

One of the advantageous implications of the present invention is thatthe inorganic N₂- and/or O₂-containing gases that are used show a minorinteraction with the polymer per se, if any. Many commercial blowingagents, such as volatile hydrocarbons (e.g. ethane, propane, ethylenepropylene), volatile halogenated hydrocarbons (e.g. methyl fluoride) orcarbon dioxide, dissolve to some extent into the polymer matrix. Thismeans that for some applications they will have to be removed witheffort in view of potential negative effects, e.g. in the field ofsafety, health or toxicology. Such disadvantages are eliminated by usingthe present invention. The skilled man will appreciate that every gaswill dissolve to some extent in the polymer matrix, but the gases usedin the present invention are far less soluble in the polymer matrix thanthe commercial blowing agents mentioned above. The impregnating gas ispreferably nitrogen or air. These gases have the additional advantagethat they are effective and cheap and have no negative environmental orhealth impact.

Preferred temperature ranges for the impregnation are from 0 to 50° C.,more preferably from 10 to 30° C. Most preferably, the temperature usedis room temperature. In this way the voids in the particles are filledwith the gas without the polyvinylarene being heated such that itdeforms. Such deformation might have a detrimental effect on thestructure and properties of the voids and thereby it would have anegative impact on the expandability of the resulting impregnatedparticles. Moreover, the low temperature ensures that the particlesremain free flowing and do not stick to each other, which might occur ifthe impregnation would be conducted at higher temperatures.

The pressure under which the impregnation takes place is from 100 to2,000 kPa gauge. Preferred pressures are between 100 and 1,500 kPagauge, more preferably between 300 and 800 kPa gauge. The employment oflower pressures would mean that the voids would merely be filled withgas, e.g., nitrogen or air, at about atmospheric pressure. Such areplacement would result in an insufficient expansion, if any. Higherpressures than 2,000 kPa gauge are possible, but are undesirable fore.g. economical and safety reasons (e.g. pressure vessels must be betterequipped). Moreover, sampling becomes much more difficult.

After the impregnation the impregnated particles can be taken to aconventional expansion unit in order to be expanded to apparentdensities lower than that of the compact polyvinylarene particles. Thereis no necessity to take any specific precautions in taking the particlesto the expansion unit. However, it could be useful to do such via apressure sluice.

Accordingly, the present invention also provides a process for thepreparation of expanded polyvinylarene particles in which expandablepolyvinylarene particles are prepared by a process as described above,and the impregnated particles thus obtained are expanded to an apparentdensity lower than that of the compact polyvinylarene particles.

In the expansion process, the impregnated particles are expanded to afinal apparent density which is suitably at least 3 times lower thantheir original density. More suitably, the impregnated particles areexpanded to a final apparent density of at least 5 times lower than thatof the compact polyvinylarene particles. Although the expansion can beconducted to any density desired, it is practical to conduct theexpansion to an apparent density which is up to 20 times, morepreferably up to 40 times lower than the apparent densities of thecompact polyvinylarene particles.

The expansion process to arrive at the desired final apparent density istypically carried out in steps. Thereto, the expanded polyvinylareneparticles obtained after the expansion are advantageously againimpregnated (“re-impregnated”) with an inorganic N₂- and/orO₂-containing gas and the thus re-impregnated particles are againexpanded. The process of re-impregnation can be repeated up to a numberof times. However, the skilled artisan will strive to a balance betweenthe duration of the impregnation in order to maximise the amount ofimpregnated gas on the one hand and a low number of repetitions of theimpregnation and expansion sequence on the other. Suitably this willlead to a process in which the impregnation and expansion steps arerepeated between 1 and 4 times.

As already indicated the expansion can be conducted in any conventionalexpansion unit. This means that the expansion can be effected by the useof hot air, hot water, a hot oil bath, infra red radiation or microwaveradiation. It is preferred to use the most common method, i.e., the useof steam. Steam may be used of temperatures of 100 to 168° C. atpressures of 0 to 600 kPa gauge, depending on the presence of additivesand/or other polymers in the pre-expanded polyvinylarene particle. Inthe case of polystyrene, it is preferred to use saturated steam of atemperature of 100-125° C. at pressures of 0 to 230 kPa gauge. It ishowever observed that other commercial methods can also be used.Expansion by means of hot air having a temperature of 90-200° C., isalso suitable in particular for the first expansion step. Preferably,the air has a temperature of 95-160° C. and most preferably from100-140° C. The exposure preferably has a duration of up to 3 hours.

As already indicated in the description above, expanded particles aresuitably put into a mould and heated so that the expanded particles fusetogether to produce foamed moulded articles. Therefore the inventionfurther provides a process for preparing a foamed article in whichexpanded polyvinylarene particles obtained by a process according tothis invention are heated in a mould till the polyvinylarene particlessoften and stick together, and the heated mould thus obtained is cooledto yield a foamed article. The heating in the mould is conventional andis typically in the range from 110 to 125° C.

The invention will be illustrated by means of the following example(s).

EXAMPLES

All apparent densities were measured in accordance with the followingmethod.

A cylindrical cup of 1000 cm³+/−2 cm³ capacity, having an insidediameter of 66 mm and a height of 293 mm was weighed to the nearest 0.1gram (atmospheric pressure, room temperature). Subsequently, the cup wasfilled with polyvinylarene particles. A perfectly flat metal scrapperwas used to tap three times against the side of the cup and subsequentlyto scrape off the excess material on the top of the cup, without shakingthe cup. The material in the cup was weighed to the nearest 0.1 gram andthe weight of the polyvinylarene particles in grams of 1 cm³ wascalculated and converted to kg/m³.

Examples 1 and 2

Compact polystyrene particles were prepared by a suspensionpolymerisation process. Hereto, 4,000 gram demineralised water, 3,680gram styrene, conventional suspension stabilisers, were mixed at astirring rate of 475 rpm. The polymerisation was started by raising thetemperature to 86° C. and addition of peroxide initiators. After around6 hours, the temperature was raised to around 120° C. where it was keptduring 2 hours. After finishing the polymerisation, the reaction mixturewas cooled.

The particles had an apparent density d₁ of 650-630 kg/m³ and a particlesize in the range of 700-900 μm.

Subsequently, the compact polystyrene particles were impregnated withair by storing them in a pressure vessel at room temperature at an airpressure of 600 kPa gauge. After 16 hours, the pressure was released,the impregnated compact polystyrene particles were taken out and placedin a KURTZ KV450 batch steam expander where they were allowed to furtherexpand (KURTZ is a trademark). The expansion conditions are set out inTable I.

From the particles thus obtained, the apparent density d₂ was measured.

Subsequently, the particles were again subjected to the impregnation andexpansion procedure as described above, with the exception that thepressure was released after two hours. The resulting apparent density d₃was measured.

This procedure was repeated two more times, until a particle having anapparent density d₅ was obtained. The results are indicated in Table I.

TABLE I d₁ expansion d₂ expansion d₃ expansion d₄ expansion d₅ Ex.(kg/m³) conditions¹ (kg/m³) conditions¹ (kg/m³) conditions¹ (kg/m³)conditions¹ (kg/m³) 1 630 P: 150 kPa g. 531 P: 140 kPa g. 323 P: 140 kPag. 90 P: 80 kPa g. 20.4 T: 127 ° C. T: 126 ° C. T: 126 ° C. T: 117 ° C.t: 15 sec. t: 15 sec. t: 15 sec. t: 15 sec. 2 650 P: 165 kPa g. 537 P:140 kPa g. 324 P: 140 kPa g. 86 P: 140 kPa g. 18.8 T: 129 ° C. T: 126 °C. T: 126 ° C. T: 126 ° C. t: 15 sec. t: 15 sec. t: 15 sec. t: 15 sec.¹P = steam pressure (kPa gauge) T = steam temperature (° C.) t =steaming time (seconds)

What is claimed is:
 1. Process for the preparation of expandablepolvinylarene particles in which compact polyvinylarene particles areimpregnated by an inorganic compound selected from the group consistingof N₂- and/or O₂-containing gas at a temperature below 95° C. and at apressure of 100 to 2,000 kPa gauge.
 2. Process according to claim 1, inwhich the polyvinylarene particles are polystyrene particles.
 3. Processaccording to claim 1, in which the compact polyvinylarene particles havean apparent density of more than 600 kg/m³.
 4. Process according toclaim 1, in which the polyvinylarene particles are impregnated byexposing the particles to the inorganic N₂- and/or O₂-containing gas attemperatures ranging from 0 to 500° C. and at a pressure of 100 to 1,500kPa gauge.
 5. Process according to claim 1, in which the gas is nitrogenor air.
 6. Process for the preparation of expanded polyvinylareneparticles in which expandable polyvinlarene particles are prepared by aprocess according to claim 1, and the impregnated particles thusobtained are expanded to an apparent density lower than that of thecompact polyvinylarene particles.
 7. Process according to claim 6, inwhich the impregnated particles are expanded to a final apparent densityof at least 5 times lower than that of the compact polyvinylarene. 8.Process according to claim 6, in which expanded polyvinylarene particlesobtained after the expansion are again impregnated with an inorganic N₂-and/or O₂-containing gas and the thus impregnated particles are againexpanded.
 9. Process according to claim 6, in which the impregnatedparticles are expanded by exposing them to steam of temperatures of100-168° C. at pressures ranging from 0 to 600 kPa gauge.
 10. Processfor preparing a foamed article in which expanded polyvinylareneparticles obtained by a process according to claim 6 are heated in amould till the polyvinylarene particles soften and stick together, andthe heated mould thus obtained is cooled to yield a foamed article. 11.Process for the preparation of expandable polyvinylarene particles inwhich compact polyvinylarene particles containing voids are impregnatedby exposing said compact polyvinylarene particles to an inorganiccompound selected from the group consisting of N₂- and/or O₂ containinggas and at a temperature below 95° C. and at a pressure of 100 to 2,000kPa gauge to maintain the structure and properties of said voids in saidcompact polyvinylarene particles, wherein said inorganic N₂- and/or O₂containing gas contains a low volume of organic compounds and whereinsaid inorganic N₂- and/or O₂ containing gas, being less soluble in thepolymer matrix compared to said organic compounds, remains essentiallyin said voids of said compact polyvinylarene particles for an expansionof said particles.